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RE: Grade Beam Lateral Force Problems

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How about using Geoform on the active side to reduce stresses on the grade
beam?  See www.geofoam.com for more info.

Martin


-----Original Message-----
From: Brian S Bossley [mailto:BSBossley(--nospam--at)venturaengineering.com]
Sent: Thursday, July 28, 2005 9:40 AM
To: seaint(--nospam--at)seaint.org
Subject: Grade Beam Lateral Force Problems

I have a 7' deep x 1' wide grade beam/ retaining wall that is 3' below
ground on the passive side and 7' below grade on the active side (plus
has a surcharge of 200psf) resting on caisson foundations.  I had
originally assumed that I would restrain the top of the grade beam with
the slab and that I would reinforce the rest of it for the lower portion
of the bending due to lateral load.  Then the owner came back and said
he did not want to tie the slab into the grade beam.

So now I have a few issues: The Lateral load has to be taken out in weak
axis bending of the grade beam, I would imagine - I don't see any other
way to do it.  That's really not a problem because it's easy enough to
just reinforce the grade beam for the bending moment and it's done.  The
main problem, however, is the eccentricity due to the lateral load.  Is
that taken into the grade beam as a torque?  Or would I look at this as
a retaining wall and then take the 'bearing' all the way to the
caissons?  It will get some bearing due to the fact that it's not
completely rigid, but the caissons at the ends are much more rigid than
the beam, so I'd think the grade beam assumption still stands.  I know
people do this all the time, but I'm not sure what the most accurate way
to look at this would be.

The problem with looking at this like a grade beam is that the
'overturning moment' at the supports (caissons) will be huge. But then
the problem with designing this as a retaining wall is that in order to
get enough force to resist the overturning, the grade beam will need to
be much, much larger (heavier).

Any help would be greatly appreciated.

Brian S. Bossley, PE
Ventura Engineering
7610 Olentangy River Rd.
Columbus, OH 43235
(614) 847-1110 x121

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